1. Field of the Invention
The invention relates to a method of and a device for detecting the flatness of a metal product running along a longitudinal direction and applies especially to bands at high temperature.
2. Description of Related Art
When rolling metal bands and especially thin rolled sheets, there may be flatness defects occurring when the band has been laid to rest, but are in a latent stage when the band is under tensile load. Such defects are caused, generally, by slight variations, over the width of the band, of the elongation performed by the roll mill and may be corrected while acting on the rolling conditions and, especially, on the distribution, in the transversal direction, of the clamping pressure applied between the working rolls.
To this end, it is possible, in modern roll mills, to act on the profile of the passage gap of the product, for instance by exerting bending loads on the ends of the working rolls. One may also use a back-up roll comprising a ductile shroud mounted to rotate round a fixed shaft and resting on the said by a plurality of jacks distributed over the width of the band and that may be adjusted in position and in pressure.
These means of adjustment are operated by information provided by a measuring device, placed downstream the roll mill, and sensitive to variations, over the width of the band, of the tensile load applied on the said band, which correspond themselves to the elongation variations of the longitudinal fibres of the band.
Such a measuring device consists, usually, of a deflector roll comprising a cylindrical body mounted to rotate round an axis perpendicular to the longitudinal running direction of the band. The said band is applied under tensile load on an angular sector of the external face of the roll that is fitted with a series of sensors enabling to measure the local application pressure variations of the band. Usually, these detectors are spaced regularly from one another and distributed over the whole length of the roll, whereby the band is divided into a series of longitudinal zones each having a set width, zones on which is integrated the measurement of the latent defect to be corrected.
In a known arrangement described, for example, in the document U.S. Pat. No. 3,481,194, the deflector roll comprises a central tubular body of sufficient thickness to confer the necessary strength and fitted, on its external face, with a plurality of recesses in which are placed measuring sensors.
Generally speaking, a measuring roll comprises a plurality of detection zones distributed over its whole length and each zone is fitted with a sensor for transmitting a signal that depends on the application pressure of the corresponding portion of the band, when passing through that detection zone in the angular contact sector of the band of the roll.
Moreover, to prevent any interferences between the measurements taken on two adjacent measuring zones, the sensors are advantageously offset at an angle from one zone to the next.
In such an arrangement, each measuring sensor is sensitive to the application pressure of the band, but may also be influenced by other factors that may affect the measurements negatively.
For example, to avoid any direct contact between the sensors and the running band, each recess of a sensor is closed to the outside, by a protection wall that may consist of a thin shroud covering the whole tubular body of the roll, or of a cap-shaped part slightly ductile in order to transmit to the sensors the pressure applied by the band. Such a device is therefore sensitive to thermal deformations caused when the temperature of the roll rises.
Similarly, the measuring sensors are not provided, normally, to operate at high temperature.
Therefore, until now, flatness measuring rolls had been used in cold rolling facilities in which the rolled band could be maintained at moderate temperature.
However, the latent defects resulting from elongation variations over the width of the band also occur during hot rolling and it has appeared that as of that stage, risks of flatness defects should be avoided.
In this view it had been considered sufficient so far, to provide the roll mill with a mathematical model enabling to foresee the defect zones in order to avoid the occurrence of the said defects, as far as possible, while acting on the different means of adjustment of the roll mill. The defects to be corrected can also be determined by optical means on the product when the said is not subject to any tensile load, i.e. on the head of the band, before winding the band around the coiler. Such a system therefore does not enable to control the flatness of the product over its whole length and it is thus preferable to detect the latent defects as soon as the band comes out of the roll mill.
To this end, it has been suggested in the document EP-A-0.858.845, to place in a hot rolling line, a measuring roll in which the band can be applied under tensile load.
However, even in the case of a non-ferrous metal such as aluminium, the hot band is at high temperature, which affects negatively the measurements that must be compensated for. For example, the roll can be calibrated at various temperatures in order to make the necessary corrections in relation to the temperature of the band, but such a calibration is not easy to perform.
The purpose of the invention is to remedy these problems thanks to particularly simple arrangements that can be applied to all types of flatness measuring rolls and that enable to avoid calibration of the sensors in relation to various operating temperatures.
The invention applies therefore, generally, to a method of and a device for detecting the flatness of a band product in which the band is subject to tensile load and applied on the angular sector of a measuring roll mounted to rotate round an axis perpendicular to the longitudinal running direction of the band and having a cylindrical external face comprising an angular contact sector with the band and a free sector.
According to the invention, the roll is cooled down forcibly by circulating a heat exchanging fluid along at least one portion of the free sector of the external face of the roll and one determines the parameters responsible for the cooling efficiency such as the opening of the angular cooling sector along which the fluid circulates, the initial temperature of the said fluid and the circulation flow rate, so that, after heating up while passing through the sector in contact with the band, the external face of the roll is brought back, after passing through the cooling sector of the roll, to a pre-set equilibrium temperature.
In a first particularly advantageous embodiment, forced cooling of the roll is performed by spraying a heat exchanging fluid by means of spray ramps distributed over at least one portion of the free sector of the roll and one adjusts at least the temperature of the fluid and the spray flow rate in relation to the temperature of the band, the running speed and the thermal exchange conditions, in order to bring back to a set level the temperature of the external face of the roll immediately before it goes through the contact sector.
In another embodiment, the flatness measuring roll being located beneath the band, the external face of the said roll comprises a lower section immersed in a heat exchanging fluid bath provided in a tub situated beneath the roll and associated with means for circulating the liquid with an adjustable flow rate between an inlet orifice and an outlet orifice of the tub, and one adjusts at least the initial temperature of the liquid as it reaches the bath and the circulation flow rate, in order to bring back to a set level the temperature of the external face of the roll immediately before it goes through the contact sector.
Thanks to these arrangements, the external face of the roll is brought back, before it goes through the contact zone, to an equilibrium temperature t that is linked with the temperature of the band t1 and the initial temperature t2 of the heat exchanging fluid by a formula such as:   t  =                    a        ⁢                  A                ⁢                  t          1                    +              b        ⁢                  B                ⁢        t2                            a        ⁢                  A                    +              b        ⁢                  B                    
in which a is the thermal exchange coefficient between the band and the roll, b is the thermal exchange coefficient between the heat exchanging fluid and the roll, A the angular contact sector and B the angular cooling sector. According to the invention, one can act, during operation, on at least one of the parameters of the said formula in order to maintain the equilibrium temperature t at a constant level.
The invention also covers a device for flatness detection for implementing the method, comprising a means for forced cooling of the external face of the measuring roll by circulating a heat exchanging fluid along at least one portion of the free sector of the roll and means for adjusting the cooling conditions in order to maintain the external face of the roll at a set equilibrium temperature, with controlled cooling of each detection zone as it goes through the free sector of the roll.
The invention applies especially to the flatness measuring rolls of the type comprising a plurality of detection zones distributed over the length of the roll and each fitted with a sensor for transmitting a signal related to the application pressure of a corresponding portion of the band, when the said detection zone passes through the angular sector where the band contacts the roll, whereas the said detection zones are brought back to the same equilibrium temperature each time it passes through the free sector of the roll.